Polyethylene (PE) is synthesized by polymerizing ethylene (CH2═CH2) monomers. Because it is cheap, safe, stable to most environments and easy to be processed, polyethylene polymers are useful in many applications. According to the properties, polyethylene can be classified into several types, such as but not limited to LDPE (Low Density Polyethylene), LLDPE (Linear Low Density Polyethylene), and HDPE (High Density Polyethylene). In another classification, the used polyethylene can be classified as Ultra High Molecular Weight (UHMW), High Molecular Weight (HMW), Medium Molecular Weight (MMVV) and Low Molecular Weight (LMW). Each type of polyethylene has different properties and characteristics.
Ethylene polymerization processes are frequently carried out in a loop reactor using ethylene monomer, liquid diluent and catalyst, optionally one or more co-monomer(s), optionally an activating agent or co-catalyst and optionally hydrogen. The polymerization in a loop reactor is usually performed under slurry conditions, with the produced polymer usually in the form of solid particles which are suspended in the diluent. The slurry in the reactor is circulated continuously with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent. Polymer slurry is discharged from the loop reactor. After the polymer product is collected from the reactor and the hydrocarbon residues are removed, the polymer product is dried, additives can be added and finally the polymer may be mixed and pelletized. The resulting product can then be used for the manufacturing of various objects.
Polymerization of polyolefin such as polyethylene can also be performed in continuous stirred tank reactors (CSTR). These polymerizations in CSTR are usually performed in the presence of Ziegler-Natta catalysts. Metallocene catalysts are not preferred because of lower intrinsic activity combined with shorter life time and higher sensitivity to contaminants. Intrinsically Ziegler-Natta catalysts lead to poorer product properties due to multiple site behavior. Additionally, Ziegler-Natta catalysts have limited comonomer response and lead to significant amount of polymer chains dissolved in the continuous phase.
Therefore, it is an object of the present invention to provide a process for preparing polyethylene prepared in at least one continuous stirred tank reactor having improved properties.